Field of the Invention
The present invention is directed to controllers and more particularly to compensation circuits which react to load changes.
There are a wide variety of controllers used to supply power to various types of loads. Controllers used to supply power to multiple lamps may include constant current or constant voltage types of controllers. Problems are encountered, however, when one lamp of a parallel connected group of lamps fails. The decrease in voltage drop across the line when a lamp fails increases the voltage across the remaining lamps. A constant voltage source, because it is incapable of recognizing that one of the lamps has failed, continues to supply the same voltage to the line plus the lamp load resulting in a decreased voltage drop across the line resistance and an increased voltage across the lamp load. This may cause a premature failure of the remaining lamps.
A constant current source suffers from an even greater problem. When a lamp of a parallel connected group of lamps fails, the current drawn by the lamp group decreases. The constant current source, sensing a drop in current, increases its output voltage to maintain a constant current output. In that situation, the remaining lamps must dissipate the additional power resulting from the increase in the voltage output from the controller.
One attempt at providing a failure compensation circuit is disclosed in U.S. patent application Ser. No. 33,543, filed herewith and which is assigned to the same assignee as the present invention. Disclosed in that application is a failure compensation circuit which uses a current transformer to provide an input signal representative of the current delivered to the load. Whenever a portion of the load drops out, the current drawn by the remaining load decreases. An average value of the new current is used to automatically reduce the voltage delivered by a power supply. Because an average value of the input signal is used to modify the voltage delivered by the power supply, it takes a predetermined period of time , for example, five cycles, before the power supply actually reduces the voltage supplied to the lamps.
Experiments have shown that when a lamp fails, arcing (a plasma discharge within the lamp) may occur which causes a current spike. It is also known that as lamps age, their sensitivity to over-voltage increases. Aged lamp failures often occur too quickly to be prevented by the failure compensation circuit discussed above.
Illustrated in FIG. 1 is a current versus time graph of the current drawn by a parallel connected group of lamps. The first spike was generated by a lamp which failed after 780 hours of use. That spiked triggered the failure of three other lamps which had 780, 780, and 0 hours of use. The subsequent failures occurred within five 60 Hz AC cycles of the first current spike.
Thus, there is a need for a lamp controller capable of recognizing when a lamp of a parallel connected group of lamps has failed for substantially instantaneously reducing the power output to the remaining lamps to prevent their failure.